Unfortunately this mod is unsupported and from years ago and it was still in beta back then. Apparently it works with 3.8 but it's not recommended for obvious reasons. It'd be great if this could be included as standard as some users may only want to search for new posts within certain forums, and to keep this perpetual.
Example of use - forums with forum game forums. I think that 'forum games' forums get in the way of real discussion topics. I'd like to be able to opt-out of the forum games forum so it doesn't show in my new posts listing. Or perhaps a football forum - members who are not into sports may not want to see these threads appearing in new posts. This is easily done in the UserCP.